Mouse outer medullary collecting duct (OMCD) cells possess a K_ATP channel

The OMCD is known to possess an active apical H⁺-K⁺ATPase and a basolateral Na⁺-K⁺ pump. Homeostatic adaptive increases in the activities of both transporters in the OMCD have been reported in potassium-deficient and potassium-loaded animals, respectively (Wingo, AJP 253:F1136, 1987; Doucet & Katz, AJP 238:F380, 1980). These adaptive mehanisms imply that the OMCD experiences significant K⁺ fluxes and raises the physiological need for K⁺ exit pathways. Previous whole-cell patch experiments in rabbit OMCD cells in primary culture reported a small K⁺ conductance (Pappas & Koeppen, AJP 263:F1004, 1992). Moreover the ROMK1 isoform of the K_ATP channel has been demonstrated in rat OMCD segments by in situ hybridization (Lee & Hebert, AJP 268:F1124, 1995). Nevertheless, results of single channel studies have not been reported. The aim of this study was to identify K⁺-selective channels in the OMCD. Using the patch clamp technique, we have demonstrated K⁺-selective (K⁺:Na⁺ = 20:1) channels in the apical membrane of OMCD, cells, a cell line previously characterized by Hays & Alpem (JASN 3:779, 1992). In symmetrical (140 mM KCI) solutions, the single channel conductance was 67.0 ±4.3 pS for inward currents and 34.3 ±2.3 pS for outward currents. In the excised (inside-out) state, the channel was sensitive to intracellular barium (2 mM) but insensitive to voltage and micromolar [Ca²⁺]. The NP₀ was reduced from 0.60 ±0.06 to 0.05 ±0.03 with the addition of ATP (2 mM) and returned to 0.53 ±0.04 after removing ATP (n=3). These K_ATP channels may be the equivalent of the ROMK1 isoform in rat OMCD segments, and could serve as apical K⁺-exit pathways in the OMCD, to accomplish K⁺ secretion and/or K⁺ recycling.